The present invention relates to electro-optic devices and systems, and more specifically, to photonic systems for performing selective sideband amplification, conversion from phase modulation to amplitude modulation, frequency multiplication, signal up- and down-conversion, and coherent optical signal processing.
Acoustic waves can cause variations of the density of a medium in which they travel. The density variations can effect optical gratings. Scattering of an electromagnetic wave by such acoustic gratings in an optical medium is known as xe2x80x9cBrillouin scatteringxe2x80x9d. The frequency of the scattered electromagnetic wave in the Brillouin scattering is shifted with respect to that of the original electromagnetic wave due to the Doppler effect by the motion of acoustic waves.
Energy is exchanged in the Brillouin scattering between the optical medium and the electromagnetic wave. Depending on the relative directions of the acoustic wave and the electromagnetic wave, the frequency of the scattered electromagnetic wave may be down-shifted to a lower frequency (i.e., xe2x80x9cStokes shiftxe2x80x9d) or a higher frequency (i.e., xe2x80x9canti-Stokes shiftxe2x80x9d).
Stimulated Brillouin scattering (xe2x80x9cSBSxe2x80x9d) is a nonlinear optical effect which occurs when a coherent electromagnetic wave with an intensity above a certain threshold level is used as a pump in a Brillouin scattering process. See, for example, A. Yariv, Chapter 18, Quantum Electronics, 2nd ed., 1975 (John Wiley and Sons). The nonlinear interaction of the optical medium and the coherent optical pump wave at a frequency xcexdp generates an acoustic wave due to the electrorestrictive effect. This acoustic wave forms a moving acoustic grating in the medium which moves in the same direction of the optical pump wave. The grating scatters the pump wave.
In general, multiple scattered waves are generated by the grating. However, due to the phase-matching restraints, the strongest scattered wave is the back-scattered wave which propagates in the opposite direction of the pump wave. Thus, the frequency xcexdB of the back-scattered optical wave from the acoustic grating is down-shifted relative to the pump wave frequency xcexdp by xcexdD=2n"ugr"a/xcexp due to the Doppler effect, where "ugr"a is the velocity of the acoustic wave in the medium, n is the refractive index of the medium, and xcexp is the wavelength of the optical pump wave. The remainder of the input pump wave transmits through the medium.
When the input optical power exceeds the SBS threshold, a significant portion of the input power is transferred into the back-scattered optical wave. This results in a saturation behavior in the transmitted wave, i.e., the power of the transmitted wave will no longer increase linearly with the input power. The SBS threshold is known to be linearly proportional to the spectral linewidth of the optical pump wave. Therefore, an optical pump wave with a narrow linewidth can be used to reduce the SBS threshold. For example, in many commercial silica fibers, a SBS threshold of several milliwatts may be achieved by using a pump wave at about 1.3 xcexcm.
When a narrow-band seed signal in the opposite direction of the pump wave with the same frequency of the back scattered wave at xcexdB=(xcexdpxe2x88x92xcexdD) is injected into an optical medium, the interaction between the seed signal and the pump wave can significantly enhance the acoustic grating initially induced by the pump wave. This effect, in turn, increases the back-scattering of the pump wave into the seed signal and thereby effectively amplifies the seed signal. Therefore, the influence of the seed signal converts the spontaneous Brillouin scattering into a stimulated Brillouin scattering, at a pump power much below the SBS threshold. The stimulated back scattering light adds up in phase with the seed signal. This process is known as Brillouin amplification. See, for example, G. P. Agrawal, Nonlinear Fiber Optics, Academic Press, San Diego (1989), Chapter 9.
Brillouin scattering and amplification in optical fibers has been investigated for optical communication applications. See, for example, Olsson and Van Der Ziel, xe2x80x9cCharacteristics of a semiconductor laser pumped Brillouin amplifier with electronically controlled bandwidthxe2x80x9d, Journal of Lightwave Technology, Vol. LT-5, No. 1, pp. 147-153 and Tang, xe2x80x9cSaturation and spectral characteristics of the stokes emission in the stimulated Brillouin processxe2x80x9d, Journal of Applied Physics, vol. 37, pp. 2945-2955 (1966). Stimulated Brillouin processes was considered by many as unsuitable for digital fiber optical communication systems at least partially due to its narrow gain bandwidth and high spontaneous emission noise. Many conventional optical communication systems are designed with provisions to suppress the Brillouin scattering in fibers as noise.
The present invention includes a use of the Brillouin scattering in an optical medium to effect selective sideband amplification for photonic devices and systems.
One embodiment of the invention includes a first signal light source that generates an optical carrier signal, a modulator for imposing information on the carrier signal and generating sidebands around the carrier frequency, an optical medium having an electrorestrictive effect, and a second pump light source that generates an optical pump signal in the opposite direction of the carrier signal within the optical medium. The frequency of the Brillouin signal is tuned to align with a selected frequency sideband or to be near the selected sideband within the bandwidth of the Brillouin gain profile to amplify the selected sideband.
Photonic RF signal mixing can be achieved by using the Brillouin selective sideband amplification. A sideband around a carrier frequency in a carrier signal produced by a modulation of a local oscillator can be aligned with the Brillouin signal to be amplified by tuning the relative frequency between the pump light source and the signal light source. Since the down or up converted signal is the beat between the amplified local oscillator sideband and a signal sideband, the converted signal is also amplified. A conversion gain can be realized when the selectively amplified local oscillator sideband is larger than the carrier signal by the amplification. In addition, when a higher order location oscillator sideband is selectively amplified, a harmonic signal up- or down-conversion can be effected. Furthermore, the amplified sideband is automatically phase and frequency locked to the signal light source with a stability determined by the modulation driving signal.
Another aspect of the Brillouin selective sideband amplification is conversion of a phase modulation into an amplitude modulation by selectively amplifying a phase-modulated sideband. Since amplitudes of the amplified sideband and the opposite-phase counter part on the other side of the carrier signal are no longer the same, the beat signals between the symmetric sidebands and the carrier signal are only partially canceled to produce a net amplitude modulation signal. High phase-to-amplitude conversion can be achieved with this scheme due to the Brillouin gain. When a high order sideband in a phase-modulated signal is selectively amplified, a beat signal between the sideband and the carrier signal has a frequency that is a multiplication of the phase modulation frequency by a factor of the order number of the amplified sideband. This effects a frequency multiplication.
Yet another aspect of the Brillouin selective sideband amplification is pulse generation and manipulation. This may achieved, for example, by producing multiple Brillouin signals with multiple pump light sources at different wavelengths in resonance with the in-phase modulation sidebands in a carrier signal. The in-phase modulation sidebands are amplified due to the Brillouin gain and are summed to produce a pulsed signal in both optical and electrical domains. When the signal light source is a mode-locked laser and the multiple Brillouin signals overlap with certain selected laser modes, amplitudes or relative phase of the selected laser modes can be altered to change the shape of the laser pulse produced by the mode-locked signal laser.
Still another aspect of the Brillouin selective sideband amplification is frequency locking of different lasers. The average optical power of the optical signals in the direction of the Brillouin scattering signal can be used to indicate the degree of the frequency alignment between the amplified sideband and the pump beam. The average power reaches a maximum value when the alignment is perfect. Therefore, this average power can be used as an error signal to control the frequencies of the pump and the signal lasers within a range with respect to each other. In one implementation, an electrical signal separator is connected to the output of a photodetector to extract the low-frequency signal components for measuring the average optical power. A control circuit is used to control the frequency of one of the pump and signal lasers to increase or maximize the average optical power. Multiple lasers may be so stabilized with respect to a reference laser.
One advantage of the invention is that only the selected sideband is amplified due to the narrow bandwidth of the Brillouin scattering signal (e.g., about 10 MHZ in fibers). This makes the system energy efficient since the pump power is not converted into the carrier signal and other sidebands. In addition, this also avoids the saturation of the optical amplifier and the receiving photodetector.
Another advantage is that the Brillouin scattering signal can be tuned by changing either the signal or the pump frequencies to select any desired sideband in the carrier signal.
Another advantage of the invention is the selectively amplified sideband is automatically phase and frequency locked to the signal laser.
Furthermore, the device and system implementation of the invention is structurally simple and provides flexibility in many applications, and in particular, for fiber-based photonic systems.
These and other aspects and advantages of the present invention will become more apparent in light of the following detailed description, the accompanying drawings, and the appended claims.